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Lower bounds for the total stopping time of $3x + 1$ iterates

Authors: David Applegate and Jeffrey C. Lagarias
Journal: Math. Comp. 72 (2003), 1035-1049
MSC (2000): Primary 11B83; Secondary 11Y16, 26A18, 37A45
Published electronically: June 6, 2002
MathSciNet review: 1954983
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Abstract: The total stopping time $\sigma_{\infty}(n)$ of a positive integer $n$ is the minimal number of iterates of the $3x+1$ function needed to reach the value $1$, and is $+\infty$ if no iterate of $n$ reaches $1$. It is shown that there are infinitely many positive integers $n$ having a finite total stopping time $\sigma_{\infty}(n)$ such that $\sigma_{\infty}(n) > 6.14316 \log n.$ The proof involves a search of $3x +1$ trees to depth 60, A heuristic argument suggests that for any constant $\gamma < \gamma_{BP} \approx 41.677647$, a search of all $3x +1$ trees to sufficient depth could produce a proof that there are infinitely many $n$ such that $\sigma_{\infty}(n)>\gamma\log n.$It would require a very large computation to search $3x + 1$ trees to a sufficient depth to produce a proof that the expected behavior of a ``random'' $3x +1$ iterate, which is $\gamma=\frac{2}{\log 4/3} \approx 6.95212,$occurs infinitely often.

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Additional Information

David Applegate
Affiliation: AT&T Laboratories, Florham Park, New Jersey 07932-0971

Jeffrey C. Lagarias
Affiliation: AT&T Laboratories, Florham Park, New Jersey 07932-0971

Received by editor(s): February 6, 2001
Received by editor(s) in revised form: June 7, 2001
Published electronically: June 6, 2002
Article copyright: © Copyright 2002 American Mathematical Society

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